A British construction worker abducted by Islamist terrorists in northern Nigeria is "likely to have been killed at the hands of his captors", along with six other foreign workers, according to William Hague.

The foreign secretary described the killings as "an act of cold-blooded murder".

The hostages were taken on 16 February while working for a Lebanese construction company, Setraco, in Jama'are, a town about 125 miles north of Bauchi, where militant Islamists have launched numerous attacks. Ansaru, a splinter group independent from Boko Haram, the main terrorist group in northern Nigeria, claimed responsibility for the kidnapping.

It announced on Saturday it had killed "all the seven Christian foreigners" in retaliation for what it believed to be attempts by the British and Nigerian governments to rescue them.

The Foreign Office had earlier on Sunday said it was "aware of reports of the death of a British national in Nigeria" but declined to comment further, urging the media not to speculate because of the "extremely sensitive" situation.

Italy's foreign ministry has also said it believes the hostages are dead, according to Italian news agency Ansa.

The Italians said checks carried out in co-operation with the other interested countries led them to believe news of the killing of the hostages was true. The statement said: "It is an atrocious act of terrorism, against which the Italian government expresses the most firm condemnation, for which it can find no explanation, except barbarous and blind violence."

The terrorists made clear they believed these planes were to be used as part of an attempt to rescue the hostages. But the group cited as evidence of the British plan a report on a Nigerian website that was dated 23 February, two weeks earlier, which also included a quote from the British high commissioner in Nigeria, Rob Fitzpatrick, describing the presence of the planes as "routine military-to-military engagement".

Announcing the killings "in the name of Allah most beneficent most merciful", Ansaru said: "The British government sent five jet bombers, soldiers and intelligent parading in Bauchi in order to rescue them [sic]. They also arrested many people including women and killed some of them.

"By this progress the Nigeria and British government operation led to the death of all the seven Christian foreigners because the soul of a single believer is worth more than the lives of thousands of unbelievers." It was signed "Abu Usumatal Ansary (JAMBS [Ansaru] leader)".

A spokesman for the Ministry of Defence indicated the British jets were not involved in any attempted rescue and instead were being used to support French military operations against al-Qaida-affiliated groups in Mali.

"We are supporting the French in their activities in Mali, which includes transporting equipment and manpower," the MoD said.

Ansaru reportedly said a video of the killings would be posted online. An image accompanying the posting appeared to show a gunman standing over dead bodies. The Foreign Office declined to comment on the claim of military action or a rescue attempt.

Ansaru also claimed a message from the Nigerian president, Goodluck Jonathan, which said the government would do anything in its power to free the hostages, had contributed to the decision to kill them. On Saturday, a spokesman for the Nigerian military said at least two soldiers and 52 Boko Haram fighters died in fighting in Maiduguri following a visit by the president.

The hostage crisis started when members of Ansaru kidnapped seven employees of Setraco in a violent raid. Three of the workers were Lebanese, while the other four were from Britain, Greece, Italy and the Philippines.

Authorities said the gunmen first attacked a prison and set fire to police trucks. They then blew up a back fence at the construction company's compound and took over, killing a guard in the process, witnesses and police said.

The gunmen appeared to be organised, leaving the Nigerian household staff members unharmed while the foreigners were quickly abducted, a witness said. The group cited "the transgressions and atrocities done to the religion of Allah … by the European countries in many places such as Afghanistan and Mali".